1. Field of the Invention
The invention generally relates to WLAN (Wireless Local Area Network) receivers and receivers in other data communications systems, and in particular to the timing error correction in such receivers.
2. Description of the Related Art
A wireless local area network is a flexible data communications system implemented as an extension to or as an alternative for, a wired LAN. Using radio frequency or infrared technology, WLAN systems transmit and receive data over the air, minimizing the need for wired connections. Thus, WLAN systems combine data connectivity with user mobility.
Today, most WLAN systems use spread spectrum technology, a wide-band radio frequency technique developed for use in reliable and secure communication systems. The spread spectrum technology is designed to tradeoff bandwidth efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: frequency hopping and direct sequence systems.
The standard defining and governing wireless local area networks that operate in the 2.4 GHz spectrum, is the IEEE 802.11 standard. To allow higher data rate transmissions, the standard was extended to 802.11b that allows data rates of 5.5 and 11 Mbps in the 2.4 GHz spectrum. This extension is backwards compatible.
When operating a WLAN receiver, code synchronization is necessary because the code is the key to despreading the desired information. A good synchronization is achieved when the coded signal arriving at the receiver is accurately timed in both its code pattern position and its rate of chip generation.
Referring now to FIG. 1, the synchronization process performed in the WLAN receiver can be divided into two phases. First, a synchronization acquisition is performed in step 100 to initially synchronize the receiver with a received signal. The second part of the synchronization follows the initial acquisition since the receiver must continue to operate in such a way that it remains locked with its code reference. That is, the receiver exactly tracks in step 110 the coded incoming signal to cause its own code chip rate to match the incoming code chip rate as precisely as possible.
With respect to the synchronization algorithms used, receivers may be classified into data-aided and non data-aided receivers. The data-aided approach does not require a prior knowledge of the interference parameters but requires a training data sequence. Non data-aided (or blind) algorithms require no training data sequence but only knowledge of the desired user signal sequence and its timing.
Generally, synchronization in data communications receivers may include a frequency error correction for correcting a frequency error in the received signal, and a phase error correction for correcting the phase error that may still exist after correcting the frequency error. Moreover, data communication receivers usually apply some timing error correction to compensate for timing errors.
Timing errors can be understood as deviations in a clock's output transitions from its ideal positions. Timing errors may be induced and coupled onto a clock signal from several different sources and are usually uniform over all frequencies. Common sources for timing errors are random noise which may be electronic, thermal and mechanical noise. A well known example of such timing errors is the so called jitter. Timing errors need to be compensated since they increase the bit and packet error rates in data communications and violate the timing margins.
Most conventional timing error correction schemes in data communications receivers such as WLAN receivers make use of synchronization patterns such as synchronization preambles. A preamble is a well-chosen code sequence that is sent at the beginning of each transmission, for use at least in the synchronization acquisition process. Timing error correction schemes based on such synchronization preambles usually involve the computation of cross-correlations and therefore require involved arithmetics to be performed. Such operations can usually not be realized by simple circuit implementations.
In addition, timing error correction circuits in existing data communications receivers still have a number of problems. One problem is that conventional circuits may be unstable in operation and sometimes work unreliably. Further, the circuits often are highly involved and therefore lead to high circuit development and manufacturing costs. Moreover, instability situations may occur when performing the synchronization acquisition.